JAVA 多线程同步工具类总结

这两天看了《JAVA 并发编程实战》一书，内容很不错，关于同步常用的几个工具类总结如下：

• CountDownLatch 闭锁：可以延迟线程的进度，直到锁到达终止状态。闭锁的作用相当于一扇门，在锁到达终止状态之前这扇门一直是关闭的。当锁到达终止状态时，允许所有线程通过。CountDownLatch 有一个初始值，通过调用 countDown 可以减少该值，一直到 0 时到达终止状态。
  
  
• FutureTask 用于执行一个可返回结果的长任务，任务在单独的线程中执行，其他线程可以用 get 方法取任务结果，如果任务尚未完成，线程在 get 上阻塞。
  
  
• Semaphore 用于控制同时访问某资源，或同时执行某操作的线程数目。信号量有一个初始值即可以分配的信号量总数目。线程任务开始前先调用 acquire 取得信号量，任务结束后调用 release 释放信号量。在 acquire 是如果没有可用信号量，线程将阻塞在 acquire 上，直到其他线程释放一个信号量。
  
  
• CyclicBarrier 栅栏用于多个线程多次迭代时进行同步，在一轮任务中，任何线程完成任务后都在 barrier 上等待，直到所有其他线程也完成任务，然后一起释放，同时进入下一轮迭代。

CountDownLatch 的例子：

import java.util.concurrent.CountDownLatch;import java.util.concurrent.atomic.AtomicInteger;public class DemoOfLatch {// 利用闭锁 CountDownLatch 控制主线程和子线程的同步public static void main(String[] args) {int numberOfThread = 5;final CountDownLatch startLatch = new CountDownLatch(1);// 用于控制子线程开始final CountDownLatch stopLatch = new CountDownLatch(numberOfThread);// 用于子线程计数final AtomicInteger count = new AtomicInteger(0);// 用于分配子线程唯一标识System.out.println("Main thread start...");for ( int i=0; i<numberOfThread; i++ ) {Thread thread = new Thread( new Runnable() {@Overridepublic void run() {int tid = count.getAndIncrement();try {// 等代主线程打开启动信号startLatch.await();System.out.printf("Thread %d started...%n", tid);int duration = (int)(Math.random() * 5000);Thread.sleep(duration);} catch (InterruptedException e) {e.printStackTrace();Thread.currentThread().interrupt();} finally {System.out.printf("Thread %d stoped...%n", tid);// 线程终止前减少线程计数stopLatch.countDown();}}});thread.start();}// 在放行子线程之前做点什么别的事情System.out.println("Main thread do preparation work for child threads...");try {Thread.sleep(2000);} catch (InterruptedException e) {e.printStackTrace();}// 打开闭锁放行所有子线程System.out.println("Main thread let child threads go...");startLatch.countDown();try {// 等待子线程计数降为 0 即所有子线程执行完毕System.out.println("Main thread wait for all child threads...");stopLatch.await();} catch (InterruptedException e) {e.printStackTrace();}System.out.println("Main thread exit...");}}

FutureTask 的例子：

import java.util.concurrent.Callable;import java.util.concurrent.ExecutionException;import java.util.concurrent.FutureTask;public class DemoOfFutureTask {public static void main(String[] args) {// 创建一个 Future Task 用于并发执行长任务final FutureTask<Movie> future = new FutureTask<Movie>( new Callable<Movie>() {@Overridepublic Movie call() throws Exception {System.out.println("Future task started...");Thread.sleep(5000);System.out.println("Future task finished...");return new Movie("2012","Unknown");}});// 在子线程中启动任务Thread thread = new Thread(future);thread.start();// 主线程干点别的事情System.out.println("Now let's do sth eles...");try {Thread.sleep(1000);} catch (InterruptedException e1) {e1.printStackTrace();}// 主线程开始取结果System.out.println("Now wait for result of future task...");try {Movie res = future.get();System.out.printf("Result from task is name=%s, actor=%s", res.name, res.actor);} catch (InterruptedException e) {e.printStackTrace();} catch (ExecutionException e) {e.printStackTrace();}}public static class Movie {final public String name;final public String actor;public Movie(String name, String actor) {this.name = name;this.actor = actor;}}}

Semaphore 的例子：

import java.util.concurrent.Semaphore;import java.util.concurrent.atomic.AtomicInteger;public class DemoOfSemaphore {/** * @param args */public static void main(String[] args) {final int numOfThread = 5;final AtomicInteger count = new AtomicInteger(0);// 用于分配唯一线程标识final Semaphore semaphore = new Semaphore(numOfThread); // 用于控制并发线程数目for (int i = 0; i < 10; i++) {Thread thread = new Thread(new Runnable() {@Overridepublic void run() {int tid = count.getAndIncrement();try {// 等待直到取得信号量System.out.printf("Thread %d wait on semaphore...%n", tid);semaphore.acquire();// 取得信号量之后做点事情System.out.printf("Thread %d get semaphore...%n", tid);int duration = (int)(Math.random() * 5000);Thread.sleep(duration);} catch (InterruptedException e) {e.printStackTrace();} finally {// 做完后释放信号量System.out.printf("Thread %d release semaphore...%n", tid);semaphore.release();}}});thread.start();}}}

CyclicBarrier 的例子：

import java.util.concurrent.BrokenBarrierException;import java.util.concurrent.CyclicBarrier;public class DemoOfBarrier {public static void main(String[] args) {final int numOfThread = 2;final int numOfIteration = 2;// 创建一个用于线程同步的 Barrier 对象final CyclicBarrier barrier = new CyclicBarrier(numOfThread,new Runnable() {// 当所有线程到达 Barrier 后会执行这个任务// 任务在第一个 到达 Barrier 的线程中执行@Overridepublic void run() {long tid = Thread.currentThread().getId();// 当所有线程完成一轮迭代之后做点清除/准备/提交工作System.out.printf("[%d] - All threads arrived barrier...%n", tid);try {Thread.sleep(2000);} catch (InterruptedException e) {e.printStackTrace();}System.out.printf("[%d] - Clear work done...%n", tid);}});// 创建并启动多个线程，他们在 Barrier 上同步for (int i = 0; i < numOfThread; i++) {Thread thread = new Thread(new Runnable() {@Overridepublic void run() {long tid = Thread.currentThread().getId();for ( int k=0; k<numOfIteration; k++ ) {try {// 线程进行一轮迭代，做点事情System.out.printf("Thread %d start its work...%n", tid);long duration = (int)(Math.random()*5000);Thread.sleep(duration);// 做完迭代后等待其他线程完成迭代System.out.printf("Thread %d wait on barrier...%n", tid);int num = barrier.await();// 显示完成的顺序System.out.printf("Thread %d pass barrier with order=%d...%n", tid, num);} catch (InterruptedException e) {e.printStackTrace();Thread.currentThread().interrupt();} catch (BrokenBarrierException e) {e.printStackTrace();}}}});thread.start();}}}